BaM/Pt multilayered films with fine grains and large perpendicular magnetic anisotropy for high density recording media

Abstract

A magnetoplumbite type of Ba ferrite (BaM) thin layer was deposited on a 9 nm thick Pt underlayer, and excellent c-axis orientation was observed even for an 8 nm thick BaM layer, which corresponds to only three or four BaM unit cells. The grain size was almost in the same range of 60–85 nm even when the BaM layer thickness tBaM decreased from 60 to 17 nm, and tBaM should be reduced below 10 nm to make a grain size smaller than 50 nm. However, the perpendicular coercivity Hc⊥ and squareness S⊥ decreased drastically from 2.6 to 0.5 kOe and from 0.6 to 0.2, respectively, with the decrease of tBaM from 60 to 8 nm because of higher demagnetizing field and susceptibility to thermal fluctuation. On the other hand, the [BaM(5–24 nm)/Pt(9 nm)]3 multilayer exhibited higher Hc⊥ and larger S⊥ than the BaM/Pt bilayer for the same BaM layer thickness and Hc⊥ and S⊥ of the [BaM(8 nm)/Pt(9 nm)]3 multilayer was 2.0 kOe and 0.6, respectively. It was clarified that the deposition of the BaM/Pt multilayer was very effective for achieving a high perpendicular magnetic anisotropy constant even for the ultrathin BaM layer.